The present invention relates to thermal ink jet printers, and more particularly, to a bubble-generating heating element for the print head of such a printer. It is a major object of the present invention to provide a relatively simple heating element with improved life properties.
Thermal ink jet printers, sometimes referred to as bubble jet printers, are very effective because they produce high velocity droplets and permit very close nozzle spacing for printing relatively high numbers of spots or pixels per inch on paper or other recording medium. The greater the number of pixels per inch, the better the printing resolution, thus yielding higher print quality.
Ink jet printers are usually divided into two basic types, continuous stream and drop-on-demand ink jets. Continuous stream ink jet printers expel ink whether or not ink is desired on the target portion of the recording medium. Accordingly, they typically require an electrode assembly to guide the ink stream and a gutter system to collect and recirculate the portion of ink deflected from the recording medium by the electrode assembly at points where the recording medium is to be left blank.
In contrast, "drop-on-demand" printers propel ink only when ink is required at the target portion of the recording medium. Such printers do not require an ink-recovering gutter or deflecting electrodes. Thus, drop-on-demand printers can be much simpler than the continuous stream type.
Thermal ink jet printers form one class of drop-on-demand printers, the other major class using a piezoelectric transducer to produce pressure pulses selectively to expel ink droplets. Thermal ink jet printers use thermal energy, usually supplied by the momentary heating of a resistor to produce a vapor bubble to propel the ink.
Thermal ink jet printers typically have a print head mounted on a carriage which traverses back and forth across the width of a step-wise movable recording medium. The print head generally comprises a vertical array of nozzles which confronts the recording medium. Ink-filled channels connect to an ink reservoir so that as ink in the vicinity of the nozzles is depleted, it is replaced from the reservoir. Bubble-generating resistive heating elements in the channels near the nozzles are individually addressable by current pulses representative of digitized information or video signals so that each droplet expelled and propelled to the recording medium prints a picture element or pixel.
The current pulses are applied to the heating elements to vaporize momentarily the ink in contact therewith and form a bubble for each current pulse. Ink droplets are expelled from each nozzle by the growth of the bubbles which causes a quantity of ink to bulge from the nozzle and break off into a droplet at the beginning of bubble collapse. As the bubble begins to collapse, part of the ink still in the channel between the nozzle and the bubble starts to move towards the collapsing bubble, causing a necking down of the ink near the nozzle and resulting in the separation of the bulging ink as a droplet.
The heating element is conventionally a resistor covered by one or more passivation layers. The heating element can be fabricated on a silicon substrate having a silicon dioxide layer, which serves as a thermal barrier. The resistor can be deposited on the substrate using standard thin-film processing techniques. The resistor can be a layer of an alloy of tantalum and aluminum (TaAl) and be up to several hundred microns (.mu.m) square.
Heating element thicknesses have been on the order of a few thousand angstroms (.ANG.), most of which is due to passivation layers and metallic coatings. The resistor material itself has been on the order of 1000 .ANG. or less. A trend toward lesser thicknesses, as low as 200 .ANG. is due, in part, to increased resistances and thereby to lower current driver requirements.
The relatively quiet operation perceived by the user of a thermal ink jet printer belies a violent environment on the scale of the bubble-generating resistor. The bubble-generating resistor is typically heated by current pulses. On the scale of the resistor, the shock of the bubble collapsing is a serious source of mechanical fatigue. The problem with fatigue is aggravated in printers provide for burst mode operation in which droplets can be propelled at around 50 kHz.
In addition to the mechanical shock produced by the collapsing bubbles, the resistor is subject to thermal fatigue when it is switched on and off at high frequencies. Thermal fatigue is suspected to aggravate a crack nucleation process, eroding the structural integrity of the resistor. Extended burst-mode operation can also cause heat accumulation, compounding the problem with thermal fatigue. Finally, the turbulent ink and vapor can be quite corrosive, subjecting the resistor to corrosion and erosion. Yet, given present day commercial requirements, a resistor is expected to deliver 40 to 200 million droplets before failure.
While understanding of the individual and interactive effects of the electrical, mechanical, thermal and chemical environment on the bubble-generating resistor is far from complete, the conventional wisdom is that the bare resistor material, including any layer formed thereon by exposure to air, succumbs to fatigue too easily to meet rising commercial expectations for reliability. Accordingly, the resistor material is protected by one or more passivation layers. For example, a TaAl resistor can be coated with a layer of silicon nitride, silicon carbide, or, more commonly, both to minimize the problems due to pinholes. In addition, an overcoat of tantalum or other metal is applied over the passivation layers as an additional impact buffer and as a means for evacuating leakage current. The additional layers reduce the intensity of the impact stress wave induced by the collapsing bubble on the resistor, which is thus protected from cavitation damage.
The improved corrosion and fatigue performance afforded by the additional layers is important enough so that they are applied despite a number of disadvantages. The most apparent disadvantage is the additional manufacturing complexity involved. Typically, seven film layers are required, as opposed to two for the uncoated resistor structure: correspondingly, five, as opposed to two, masking steps are required. The increased manufacturing complexity also corresponds to increased costs and decreased yields on a per wafer basis.
The passivation layers also impede the dissipation of heat. On the one hand this aggravates heat accumulation. Especially during multi-drop half-tone printing in which the burst mode is used repeatedly, accumulated heat can affect ink viscosity significantly. Ink viscosity is a critical variable in determining droplet size and velocity. Without additional complexity involved in implementing viscosity compensation schemes, the variable viscosity decreases printing precision, lowering the overall quality of the output. Furthermore, substantial heat accumulation will increase stress levels in the various layers, which can increase the failure rate of the bubble-generating resistor.
Another disadvantage of passivated resistors is that the turn-on voltage varies sensitively with passivation thickness. This makes it more difficult to determine the proper driving voltage for a given resistor. Driving the resistor with too low a voltage can result in insufficient bubble formation, while excessive voltage rapidly diminishes resistor life due to excessive heating.
An object of the present invention is to provide the best of two resistors: the fatigue resistance of a passivated resistor along with the simplicity, economy, and improved electrical and thermal characteristics of an unpassivated resistor.